Did a little bit of research into 5th wheelers a few months ago.
That one in the picture on the face of it looks ok, at 7ft wide it comes in under NSW maximun width
2.4 or 2.5m (can't remember of the top of my head). Length and height are ok.
The main things to be aware of
Lighting - easy to bring up to ADR.
Weight - at nearly 2,300kg + horses + feed + saddles + all the grog you'll need to drink while watching the missus doing something thats boring you shitless you have to make sure that the GTM
of your tow vehicle is high enough to be legal.
Axles & Brakes - some 5th wheelers have braking systems and axle & spring setups don't meet ADR's and need a full upgrade.
Hitch - basically the same as axles and brakes, some don't meet ADR. Then there's the problem getting it from Port Kembla if you haven't got the right hitch on the tow vehicle.
Engineer - imported 5th wheel caravans need an engineers report so I would assume the same for a horse float, so if I were you I would be trying te get ALL the specs of any trailer your considering and talking to an engineer before purchasing.
I'm no expert on these things but what I've written above is what I found out when researching 5th wheel caravans.